Spinal Muscular Atrophy (SMA), in its broadest sense, describes a collection of inherited and acquired central nervous system (CNS) diseases characterized by motor neuron loss in the spinal cord and brainstem causing muscle weakness and atrophy. The most common form of SMA is caused by mutation of the Survival Motor Neuron (SMN) gene, and manifests over a wide range of severity affecting infants through adults.
Infantile SMA is one of the most severe forms of this neurodegenerative disorder. The onset is usually sudden and dramatic. Some of the symptoms include: muscle weakness, poor muscle tone, weak cry, limpness or a tendency to flop, difficulty sucking or swallowing, accumulation of secretions in the lungs or throat, feeding difficulties and increased susceptibility to respiratory tract infections. The legs tend to be weaker than the arms and developmental milestones, such as lifting the head or sitting up, cannot be reached. In general, the earlier the symptoms appear, the shorter the lifespan. Shortly after symptoms appear, the motor neuron cells quickly deteriorate. The disease can be fatal and has no known cure. The course of SMA is directly related to the severity of weakness. Infants with a severe form of SMA frequently succumb to respiratory disease due to weakness in the muscles that support breathing. Children with milder forms of SMA live much longer, although they may need extensive medical support, especially those at the more severe end of the spectrum. Disease progression and life expectancy strongly correlate with the subject's age at onset and the level of weakness. The clinical spectrum of SMA disorders has been divided into the following five groups:
(a) In Utero SMA (Type 0 SMA; before birth): Type 0, also known as very severe SMA, is the most severe form of SMA and begins before birth. Usually, the first symptom of type 0 is reduced movement of the fetus that is first seen between 30 and 36 weeks of the pregnancy. After birth, these newborns have little movement and have difficulties with swallowing and breathing.
(b) Infantile SMA (Type 1 SMA or Werdnig-Hoffmann disease; generally 0-6 months): Type 1 SMA, also known as severe infantile SMA or Werdnig Hoffmann disease, is the very severe, and manifests at birth or within 6 months of life. Patients never achieve the ability to sit, and death usually occurs within the first 2 years without ventilatory support.
(c) Intermediate SMA (Type 2 SMA; generally 7-18 months): Patients with Type 2 SMA, or intermediate SMA, achieve the ability to sit unsupported, but never stand or walk unaided. The onset of weakness is usually recognized some time between 6 and 18 months. Prognosis in this group is largely dependent on the degree of respiratory involvement.
(d) Juvenile SMA (Type 3 or Kugelberg-Welander disease; generally >18 months): Type 3 SMA describes those who are able to walk independently at some point during their disease course, but often become wheelchair bound during youth or adulthood.
(e) Adult SMA (Type 4 SMA): Weakness usually begins in late adolescence in tongue, hands, or feet then progresses to other areas of the body. The course of adult disease is much slower and has little or no impact on life expectancy.
The SMA disease gene has been mapped by linkage analysis to a complex region of chromosome 5q. In humans, this region has a large inverted duplication; consequently, there are two copies of the SMN gene. SMA is caused by a mutation or deletion of the telomeric copy of the gene (SMN1) in both chromosomes, resulting in the loss of SMN1 gene function. However, all patients retain a centromeric copy of the gene (SMN2), and its copy number in SMA patients has been implicated as having an important modifying effect on disease severity, i.e., an increased copy number of SMN2 is observed in less severe disease. Nevertheless, SMN2 is unable to compensate completely for the loss of SMN1 function, because the SMN2 gene produces reduced amounts of full-length RNA and is less efficient at making protein; though it does so in low amounts. More particularly, the SMN1 and SMN2 genes differ by five nucleotides; one of these differences, a translationally silent C to T substitution in an exonic splicing region, results in frequent exon 7 skipping during transcription of SMN2 mRNA. As a result, the majority of transcripts produced from SMN2 lack exon 7, SMNΔEx7, and encode a truncated protein that has an impaired function and is rapidly degraded. The SMN protein is thought to play a role in RNA processing and metabolism. Its best characterized function is regulating the assembly of a specific class of RNA-protein complexes called snRNPs. SMN may have other functions in motor neurons, however its role in preventing the selective degeneration of motor neurons is not known.
In most cases, a diagnosis of SMA can be made on the basis of clinical symptoms and by the SMN gene test, which determines whether there is at least one copy of the SMN1 gene by detecting its unique sequences (that distinguish it from the almost identical SMN2) in exons 7 and 8. However, other forms of spinal muscular atrophy are caused by mutation of other genes, some known and others not defined. In some cases, when the SMN gene test is not possible, or does not show any abnormality, other tests such as an electromyography (EMG) or muscle biopsy may be indicated.
Medical care for SMA patients is supportive, including, respiratory, nutritional and rehabilitation care; there is no drug known to otherwise alter the course of the disease. Current treatment for SMA consists of prevention and management of the secondary effect of chronic motor unit loss. The major management issue in Type 1 SMA is the prevention and early treatment of respiratory infections, pneumonia is the cause of death in the majority of the cases. While some infants afflicted with SMA grow to be adults, those with Type 1 SMA have a life expectancy of less than two years.
As a result of the progress made in understanding the genetic basis and pathophysiology of SMA, several strategies for treatment have been explored, but none have yet demonstrated success. For example, gene replacement (of SMN1) and cell replacement (using differentiated ES cells) strategies are being tested in animals. However, these approaches to treat SMA will require many more years of investigation before they can be applied to humans. Other approaches under exploration include searching for drugs that increase SMN levels, enhance residual SMN function, or compensate for its loss.
Drugs such as indoprofen or aminoglycosides, which enhance expression of the SMN protein from SMN2 mRNA by promoting translational read-through of a stop codon, have been assessed in cell culture, but have poor central nervous system penetration. Chemotherapeutic agents, such as aclarubicin, have been shown to increase SMN protein in cell culture; however, the toxicity profile of these drugs prohibits long-term use in SMA patients. Some drugs under clinical investigation for the treatment of SMA include transcription activators, such as histone deacetylase (“HDAC”) inhibitors (e.g., butyrates, valproic acid, and hydroxyurea), the goal being to increase transcription of the SMN2 gene. However, the use of the HDAC inhibitors results in a global (nonspecific) increase in transcription and gene expression. In an alternative approach, the use of neuroprotectants that have demonstrated modest efficacy in other neurodegenerative conditions (e.g., riluzole, which is used in patients with ALS) have been chosen for investigation. Such strategies are not aimed at SMN for the treatment of SMA, but instead, are being explored to protect the SMN-deficient motor neurons from neurodegeneration.
Despite the progress made in understanding the genetic basis and pathophysiology of SMA, no therapy exists to alter the course of SMA, one of the most devastating childhood neurological diseases.